Phosphate coating of metal articles



Patented July 18, 195

2,515,934 PHOSPHATE COATING or METAL ARTICLES 1 Harry Grey Verner',(mini al, and Lindley s.

Wood, Syracuse, N. Y., assignors to Du-Lite Chemical Corporation,Middletowri-,*Conn;, a i corporation of Connecticut v r No Drawing.Application ecem b er 17,1947, p

, Serial No. 792,326

This invention'relates to the production of phosphatefcoatings on metalarticles, andrelates more particularly to a phosphatizing liquid and amethodof phosphatizing employing an organic liquid as the vehicle forthe phosphoric acidin lieuof the water of the conventional aqueousphosphatiz ing solution.

"The production of phosphate coatings on metal articles is today widelyemployed for the purpose of protecting the metal against oxidationand'providing a surface properlyconditio'ned for paintffinlshes. Inpresent phosphatizing pr'oc esses employing 'an aqueous phosphatizin'gsolution, the metal must be thoroughly clean'before phosphatiz'ing, asany rust or a 'thin film of oil or even a fingerprint may cause anunsatisfactory phosphate coating or may produce, a non-uni,- form'appearance, of the final product. Metal articles to be phosphated aretherefore usually thoroughly cleaned to remove all traces of rust, and,to afford protection during theoperations preceding phosphatizing, areusually coated with a of rust preventive oil, called slushing oil,orwith some proprietary oily product adapted to'perform asimilarprotective function. When ready" for phosphating, the metal articles are(1) degreased by solvent or vapor "d'egr'easing, this degreasing stepemploying an oil solvent such as trichlorethylene or perchlorethylene;Followingx degreasing, the metal articles are usually (2) cleanedin ahot, mildly alkaline aqueous cleaning solution, (3) thoroughly rinsed.twice in hot water, and (4) phosphatized by being either sprayed with ordipped in the hot aqueous phosphatizing solution. 'Afterthe' phosphatecoating is thus applied to the metal articles, they are (5) again rinsedin hot water, ('6) subjected to a passivation treatment in hot dilutechromic acid solution intended to prevent creeper rust under anyaccidental break in the final paint (7) again thoroughly rinsed in hotwater, and (8) dried in hot air, after which the articles are"(9)painted, usually by spraying with paint and then baking. If the paintingstep does not immediately follow the drying step, the articles must bekept dry or they may oxidize. The process above described is sometimesabbreviated by omitting the passivation treatment (step 6) and thepreceding and subsequent rinses (steps 5 and 7).

The aqueous phosphatizing solution used a process such as just describedmay comprise about 2% of a mixture offree phosphoric acid, zincphosphateand an oxidizing-agentsuch as a nitrateo'r achloratepfrequently termed an ac- 10Clairns. ((1148-4515) celerator; Thefree phosphoric acid reacts with the metal of the article and withoxygen from the, air or from the oxidizin agent or both to form anadherent coating of insoluble metallic phosphate. This phosphate coatingprotects the metal somewhat,butits primary function is to serve as abond-for'the paint finish The zinc phosphate of such an aqueousphosphatizing solution is used topirnprove the grain structure of thephosphate coating, and may act as a catalyst. The'phosphate coatingobtained improves as the temperature of the solution is raised, and thesolution is therefore used as near the boiling pointfas may,beconven'ieht.

Weh'ave discovered that improved results and great economy Qfbpera'tionmay be achieved by substitutinganor'ganic phosphatizing liquid fortheadu'eous solution above'described, while at the same time modifyingthe process in a number of particulars later discussed. The activephosphatizing agent "may be the commercial phosphoric acid known as thesyrup, and the zinc phosphateand the accelerator of the aqueoussolution'are unnecessary, although they may bej usedif desired-.-According to the preferred form of ourinvention-,-the phosphoric acidsyrup is dissolved in avolatile organic solvent, preferably"a*50 50mixture of acetone and carbon tetrachloride; The'concentration of syrupin the solventmay*range'from 1% to 7%, but we prefer 3%;a'lthough' thereis not much difierence inresults' anywhere in the range from 2% to 4%.Immediately following the conventional degreasing' step,"the organicphosphatizing liquid is applied-bydippin -or by spraying. After beingdipped or sprayed;themetal article dries in air at room temperaturealmost instantly, due to the volatility of the solvent 'employed,leaving a very thin film of metal phosphate on the metal article. "Aconventional solvent recovery system mayfiadvantageously" be employed,connected either to a hood over "the dipping tank or to the usualspraybootm thus enabling the acetone and carbon tetrachloride to berecovered and reused. The"meta1ajrticle may be painted immediately, or,if more convenient, may be stored for a reasonable period beforepainting without requiring any special precautionsin storage. Tests haveshown that polishedsteel panels so phosphated may be stored for as longas one month under ordinary "room conditions without exhibiting anysigns of rustand without impairing the subsequent paint finish. As iscustomary with aqueous phosphati'zing; the'paint is preferably sprayedon the phosphatized article and. then baked,

The extreme simplicity of this organic phosphatizing process as comparedto the aqueous phosphatizing processes now in use will be at onceapparent. The nine steps of the full aqueous process described above arereduced to three: (1) degreasing, (2) phosphatizing by dipping orspraying and practically simultaneous air drying, and (3) painting. Noelevated temperatures are required in the combined phosphatizing anddrying step. As we shall show hereinafter, it is even possible undersomecircumstances to eliminate the degreasingstep.

Metal articles phosphatized by our organic phosphatizing method, andthenpainted, Show superior results in the standard salt sprayaocelerated test as compared to similar articles produced by either thefull or the abbreviated aqueous phosphatizing methods described above.With our process, the paint does not peel along the edges of theconventional scratch, nor is there any evidence of creep. The paintremaining after the salt spray tests shows superior adhesion with ourprocess, this having been determined by subjecting the various panels toa prescribed brushing procedure.

7 While the organic vehicle we employ is more expensive than the aqueousvehicle of. former methods, the superior product obtained and thesavings effected by. the elimination of stepsand the simplification ofthe remaining steps, more than counterbalance the increased materialcost, which cost may be greatly lessenedby using conventional solventrecovery methods as above described. Moreover, the fact that our organicphosphatizing liquid may beapplied at ordinary room temperature and theaccompanying drying may also be efiected at room temperature, enablesthe necessary apparatus to be decidedly simplified, although heat may beused in our process if desired.

. In the conventional aqueous process, the hydrogen which is liberatedas a result of thereaction between the phosphoric acid and the metal,apparently tends to adhere to the metal and prevent further reaction.One purpose of the oxidizing agents or accelerators is to react withthis hydrogen and thus speed up the phosphatizing reaction by removingthe hydrogen from the reaction zone In our process. nosuch hydrogendifficulty is experienced, making it Imv necessary to employ oxidizingagents, although, as stated above, they may be used withoutdoing anyharm. We have found that zinc phosphate is entirely unnecessary in ourprocess, but, like the accelerators, it may be employed if desired. Thephosphate coating obtained by our preferred process is so much betterthan that ob.- tained by the conventional aqueous phosphatizingprocesses, that, even without the, paint finish, it affords a verysubstantial degree of protection to the metal. The chromic acidpassivation treatment, designed to prevent creep, is renderedunnecessary by the superior phosphate coating we obtain.

One of the advantages of our preferred process as described above, isthe wide applicability of the process to differentmetals. We haveobtained excellent results not only with steel articles, but witharticles made of cast iron, zinc diecast metal, aluminum-zinc die-.castmetal containing only a small proportion of zinc, and Ger-.- man silver.,Pure aluminum, and cadmiumplated articles cannot be satisfactorilyphospl a d, her by ur process or by conventional aqueous methods.

The organic vehicle employed in our process should be one in whichphosphoric acid is soluble in the proportion in which it is to beemployed, and the vehicle should preferably also be suflicientlyvolatile so that the remaining film will evaporate promptly at roomtemperature from the metal article after clipping or spraying. Thecommercial phosphoric acid syrup contains about 15% of water, and ifthis material is to be used as the source of phosphoric acid, which isdesirable from the cost standpoint, the organic vehicle employed mustalso be compatible with the resulting small amount of water in the finalmixture. It is probable also that a small amount of water is necessaryin the phosphatizing liquid in order to permit the phosphoric acid toionize.

Acetone ideally meets all of the foregoing requirements, and may be usedalone with entirely satisfactory results, but its low flash point (-100.) makes its use rather dangerous in commercial operations, creating apossible explosion hazard. We therefore prefer tomix carbontetrachloride with the acetone in order to raisethc flash po n a c asethe hazard. The carbon tetrachloride is used solely as a flashinhibitor. its function being to raise thefiash point of the mixture tothe desired extent. is necessary, carbon tetrachloride is compatiblewith acetone without destroying the ability of the acetone to dissolvethe phosphoric acid. The phosphoric acid syrup is not soluble in carbontetrachloride alone, but is sufficiently soluble in a mixture of acetoneand carbon tetrachloride.

Mixtures of acetone and carbon tetrachloride ranging from 60-40 to491-56 by volume have a fla h po nt of about54 C sl h y above the fl sPoint of ordi ary kerosene. Such mixtures may therefore beSafely'handled'with only the usual precau ons e pl yed i p t spray ngoper tions. The proportions of acetone and carbon tetrachloride may thusbe considerably varied with ut substantia y changing the flas p int, ofthe mixture. If the carbon tetrachloride b reduced o 30% of t e, m xtu ethe l h po nt dro s to &9" 0.. slightly less than that. o ke se e. a dthis would appearto be about the lower limit of safety. Acetone andcarbon tetrachloride by themse v s are completely miscible in allproporions, bu due to the wa inc dedin t e com rn roial pho phor c acidthat we p efer to emp proportion of carbon tetrachloride cannot beincreased beyond 56%, or two liqu d ases bpear. mak ng sprayin i act a Wha e een able to. o serve n c b v difi rc o o ll' asPhOSPhatiZingresults are concerned, with mixtures of acetone and .carbontetrachloride ran ing fr m 30 a b t rachl e up to 56% carbontetrachloride, as compared wi h the u e of acetone alone. For reasonsof, economy and convenience, we therefo'reprefer to use as our organicphosphatizing l quid, a mixture of equal volumes of acetone and carbontetrachloride, in which isd ssolved 3% of comm c l p sph r acid 85%syrup.

It is possible to substitute other organic liqnids forthe actone-carbontetrachloride mixture described above. We have found that 3% of commcrcil. hosph ic, c y up w l d o v inthe following vola e o ganic s lv nts:methy al ohol, amyl acetate, methyl acetate, normalrbutyl alcohol, scondary-bow alcohol. s p y alcohol, denatured ethyl c o -prowl a oohoand d xanc. A of t s solven s r 9 1 merc lly obtainable, and, l e acetoe. may be used either with or without arbon tetrachloride,

but,- 1 for the sake ;,-of'.saiety, a: flash inhibitor is eminently,desirable. rThGSe solvents are listed above in their decreasing .orderof. effectiveness, methyl alcohol and amylacetate producing a slightlyless desirable phosphate coating (judged by salt spray paint tests) thanour preferred 50-50, acetone-carbon@tetrachloride.mixture, and theothersabeingz-i successively less effective, but all ofithem produce aphosphate coating superior toa,,con,ventional aqueous:phosphatizingprocess used;-as a comparisonstandardn I v Solvents.having: a boiling point substantially higher than that of amyl acetate(148. (3.) may be regarded; for ,thepurposes of this-invention asnon-{volatile We haveg'successfully used several suchnonevolatileorganic solvents, such as octyl alcohol (B. P. 195 C.),cyclohexanol (B. P. 161.5 C.) and tri-butyl phosphate (13. P. 178 C.),in all of which phosphoricacid syrup is solubleto the extent'of 3%;These non-volatile solvents donot require a flashinhibitor, but have hedisadvantage that heatshbiild be used to accelerate the drying step, forexample, by blowing hot air over the metal articles to be dried. Theyalso produce somewhat less desirable phosphate coatings than theivolatile solvents. However, all of the nonvolatile solvents named aboveproduced better phosphate coatings (judged by salt spray paint tests).than aconventional aqueous phosphatizing process-used as a comparisonstandard, and we therefore do;-.not wish to exclude their usefrom thescopeofpur invention. I i

It is also desirable that the organic vehicle be one that has grease andoil penetrating properties. This is true of ourpreferredacetoneecarbontetrachloride m'ixturem Where theorganic vehicle has this property, itis possible to apply our phosphate coating even through a thin oil film.Thus, we have successfully coated polished steel panels dipped in eitherlubricating oil or a, com mercial slushinig oil, drained for twenty-fourhours, and then sprayed with our preferred organic phosphatizingsolvent, without cleaning or degreasing the panels in any way after theoil dip. The panels were then painted in the usual manner. Clean controlpanels were similarly treated except for the oil dip. The phosphatecoatings on the oil-dipped panels, before painting, had a poorappearance, but, judged by salt spray tests after painting, were almostthe equal of those obtained on the clean control panels. This suggeststhe possibility of eliminating the degreasing step in commercialpractice where the metal articles to be phosphatized are not excessivelycoated with oil or grease.

Our above-described low temperature, rapid process results in theformation of an extremely thin phosphate film. This film is frequentlyso thin that it would easily escape notice by the uninitiated. In theusual phosphatizing processes, the phosphate coating produced is ofsubstantial thickness, being measurable in thousandths of an inch, andis plainly evident to the naked eye. It may very well be that theexceptional results produced by our process are due in part to theextreme thinness of the resulting phosphate film. It seems quiteprobable that the adhesion between the metal and the metal phosphate andthe adhesion between the metal phosphate and the paint are both greaterthan the adhesion between superposed molecules of the metal phosphate;and, if this be true, the ideal coating would be a uniform layer ofmetal phosphate just one molecule thick. Another possible explanation isthat the thick prior art coatings are suificiently porous" to holdenough water to affect the aint adherence adversely. Regardless of thecorrectness of these theories, the fact remains,

as pointed out above, thatour process produces superior paint adherence,judged by the standardsalt spray test, as compared to previous aqueousphosphatizing methods.

Although we have thus described our invention in considerable detail inthe best form of which weare aware, in accordance with the patentstatutes, it will be evident that various changes and modifications maybe made by those skilled in the art without departing from the spirit ofour. invention. Accordingly, we desire to. be limited.

only by the scope of the appended claims.

We claim: 1

i l. The rocess of phosphate coating an article made of metal of theclass capable of reacting with phosphoric acid inform a metal phosphate,whioh lccmpriseswetting the metal article .With

a phosphatizing liquid consisting, essentially of from one to sevenpercent of concentrated ph0s-.

ing the organic solvent from the Wetted article.

by evaporation, thereby producing on the metal article a thin andsubstantially dry film of metal phosphatecapable of serving directly asa paintbonding medium. a

2 The process;as claimed in claim 1 in which the organic solvent isacetone.

i 3. Theprocess of phosphate coating an articl made of metal of theclass capable of reacting with phosphoric acid to form a metalphosphate, which comprises wetting the metal article with aphosphatizing liquid consisting essentially of from one to seven percentof concentrated phosphoric acid dissolved in a volatile organic solventtherefor, and carbon tetrachloride; said carbon tetrachloride beingpresent in sufficient amount to raise the flash point of thephosphatizing liquid materially above that of said volatile organicsolvent, said phosphatizing liquid having no aqueous phase andcontaining only a small proportion of water based on its phosphonic acidcontent; and removing the organic solvent and carbon tetrachloride fromthe wetted article by evaporation at not substantially above roomtemperature, thereby producing on the metal article a thin andsubstantially dry film of metal 131105- phate capable of servingdirectly as a paintbonding medium.

4. The process as claimed in claim 3 in which the volatile organicsolvent is acetone.

5. The process of phosphate coating an article made of metal of theclass capable of reacting with phosphoric acid to form a metalphosphate, which comprises wetting the metal article with aphosphatizing liquid consisting essentially of from two to four percentof concentrated phosphoric acid dissolved in acetone and carbontetrachloride; said carbon tetrachloride being present in sufficientamount to raise the flash point of the phosphatizing liquid materiallyabove that of the acetone, said phosphatizing liquid having no aqueousphase and containing only a small proportion of water based on itsphosphoric acid content; and removing the acetone and carbontetrachloride from the wetted article by evaporation at notsubstantially above room temperature, thereby producing on the metalarticle a thin: and substantially dryfilm of-metal phosphate capable ofserving directly as a paint-bonding. medium.

6-.- A- phosphatizing liquid for phosphate coatingaarticles: made ofmetal of the class capable of reacting with phosphonic acid to form ametal phosphate, consisting essentially of from one to seven: percent ofconcentrated phosphoric acid dissolved in an organic sol-vent therefor,said organic solvent being capable ofbeingevaporated at a moderatetemperature, and said phosphatizing liquid having no aqueous phase andcontaining only a small proportionof water based onitsphosphoricacidcontent.

7. A phosphatizing liquid for phosphate coating articles made of metalof the class capable of reacting with phosphoric acid to form a metalphosphate, consisting essentially of fromone to seven percentofconcentrated phosphoric acid dissolved in acetone, and saidphosphatizing liquid having no aqueous phase and containing only a smallproportion of water based on its phosphoric acid content.

8-. A phosphatizing liquidfor phosphate coating=v articles madeof metalof the class capable of reacting with phosphoric acid to form a metalphosphate, consisting essentially of from one to seven; percent ofconcentrated phosphoric acid dissolved in a volatile organic solventtherefor and; carbon tetrachloride, said carbon tetrachlo- 9; Aphosphatizing liquidasa claimedzin claim 8 in which the volatile organicsdlventis acetone.

102 A phosphatizing liquid for phosphate coating articlesma'd'eof;meta1- of the classcapable-oi reacting with phosphoric acid. tutor-mtaametal phosphate, consisting essentially of from two to four percentof: concentrated phosphoric acid dis solved. in acetone and carbontetrachloride; said carbon tetrachloride being present in sumcient:

; amount toaraisethe flashpointofi the phosphatizing liquid materiallyabove that of the acetone,- and said phosphatizing liquid having noaqueous phase and containing only: a small proportion of; water based;on its phosphoric acidcmrtent. GREY VERNER;

S1 WOGD;

S; G ED The following references are of record in the fiil'e ofthis'patent:

UNITED sT'A Es PATENTS Number Name Date 1,398,507 Gravell Nov. 29; 19211,428,085 Gravelle, Sept-.15; 1922" 1 ,549,442 Brown Y a Aug; n, 1925;1,592,102 Gravell- July 13; 1926 1,673,951 Rogers June 19',- 1928.1,765,331 Gravell June v1'7, 1930' 1,837,430 Gravel-l; Dec. 22; 19311,995,954 Albrecht, Mar. 26, 1935 2,142,024 Hall Dec. 27=, 1938* FOREIGNPATEN'IS Number Country Date 499,046 Great Britam o Jan. 18-, 1 939,

6. A PHOSPHATIZING LIQUID FOR PHOSPHATE COATING ARTICLES MADE OF METALOF THE CLASS CAPABLE OF REACTING WITH PHOSPHORIC ACID TO FORM A METALPHOSPHATE, CONSISTING ESSENTIALLY OF FROM ONE TO SEVEN PERCENT OFCONCENTRATED PHOSPHORIC ACID DISSOLVED IN AN ORGANIC SOLVENT THEREFOR,SAID ORGANIC SOLVENT BEING CAPABLE OF BEING EVAPORATED AT A MODERATETEMPERATURE, AND SAID PHOSPHATIZING LIQUID HAVING NO AQUEOUS PHASE ANDCONTAINING ONLY A SMALL PROPORTION OF WATER BASED ON ITS PHOSPHORIC ACIDCONTENT.